A one-year long EC-measurement took place on river K sely and tributaries in order to assess the effect of thermal water disposal on the water quality in salt concentrations. As the river catchment is an agricultural area with low precipitation in summer, it is essential to meet the EC-limits for irrigation water. The results show however that these requirements are fulfilled in non of the seasons. Background concentration levels of shallow ground waters in the Great Hungarian Plain is - according to literature – higher than the required limit. Consequently good quality irrigation water is not available.